This project explores the feasibility of inducing tolerance to tissue allografts and xenografts by engineered expression of fas ligand on tissues for transplantation. We have constructed transgenic mice in which murine fas ligand expression is directed to antigen presenting cells (APC) such as Langerhans cells by cloning into an expression vector containing the MHC class II Ealpha promoter. Four founder lines have been established and each line is being bred, backcrossed and assessed by immunocytochemistry for tissue specific expression of the transgene. Following full characterization and additional backcrossing, tissues from such mice will be transplanted onto allogeneic animals and assessed for graft rejection/ tolerance induction. An additional project involving assessment of regulatory mechanisms in expression of fas-ligand on tumor cells has sprung from our observation that fas-ligand expression on such cells is very sensitive to modulation by the expression of a bacterial phosphotransferase, mediated by a neomycin resistance gene. This aspect is of great interest because of reports that tumor cells may have enhanced survival pertaining to their expression of fas-ligand. Because of the recent findings in which expression of fas ligand on tissues for transplantation was shown to modify rejection responses, we expect that the Agency will receive clinical protocols involving use of this construct for allograft or xenograft rejection for evaluation in the near future. This project will enable the Agency to evaluate, in a scientific manner, the merits of such submissions.